By the time we're measuring the height of a BFC root, we've already
collapsed all relevant margins for the root and its descendants.
Given this, we should simply use 0 (relative to the BFC root) as the
lowest block axis coordinate (i.e Y value) for the margin edges.
This fixes a long-standing issue where BFC roots were sometimes not tall
enough to contain their children due to margins.
This patch just adds the new root paintable and updates the tests
expectations. The next patch will move painting logic from the layout
viewport to the paint viewport.
Pseudo-elements like ::before and ::after were discarded when their
content property was an empty string (ignoring whitespace), because they
are anonymous containers with no lines.
Our previous way around it was to add an empty line box (see b062a0fb7c)
however it didn't actually work for cases described in the previous
commit.
This makes avatars and cover arts square on last.fm and "fixes" the test
css-pseudo-element-should-not-be-affected-by-presentational-hints.html.
Unfortunately, this also regresses on block-and-inline/clearfix.html,
but that hopefully will be handled in subsequent commit.
